To me, there is a slight difference. To begin with, falar aos amigos sounds a lot more formal, almost poetical. Falar com os amigos is the more usual way. Also, falar aos amigos might mean to communicate something specific to them, whereas falar com os amigos might mean to converse with them. I have never heard falar aos amigos meaning to say goodbye to them, but that is only my experience.

Perhaps 'falar aos amigos' as 'cumprimentá-los', it's just used in Portugal.

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Maybe so, but that can be one of the meanings in Portugal all right. It can also mean 'to mention' or 'to discuss the matter with friends' ('vou falar (nisso) aos (meus) amigos'). 'Falar a' may also mean 'to be in friendly terms with someone'.